Dephosphorization of ferrophosphorus

ABSTRACT

A method is provided for the dephosphorization of ferrophosphorus containing about 20 to 30 percent phosphorus, with or without vanadium, chromium, nickel, manganese, etc., and the bulk of the ferrophosphorus alloy comprising essentially iron, wherein a bath of said ferrophosphorus is established containing at least one additive selected from the group consisting of carbon and silicon in amounts effective to reduce the solubility of phosphorus in said bath, thereby resulting in a substantial amount of phosphorus vapor which is removed and then recovered.

United States Patent Cosman et al.

[451 Oct. 17,1972

1541 DEPHOSPHORIZATION OF OTHER PUBLICATIONS FERROPHOSPHORUS Van Wazer,Phosphorus and its Compounds, Vol. 2 [72] Inventors: Cornelius M.Cosman, Bronxville, pp. 1,160, 1,172, 1,181, 1,194- 1,195 (InterscieneN.Y.; Donald 0. Buker, Grand 1961) Junction, Colo.

Primary Examiner-Oscar R. Vertiz [73] Asslgnee' gzz ig Climax NewAssistant Examiner-Charles B. Rodman Attorney-Kasper T. Serijan andEugene J.'Kalil [22] Filed: Dec. 24, 1969 211 App]. No.: 888,000 [57]ABSTRACT A method is provided for the dephosphorization offerrophosphorus containing about 20 to 30 percent [52] U.S.Cl..423/322,75/132 phosphorus, with or without vanadium chromium [51] Int.Cl ..C0lb 25/02, C22c 33/00 [58] Field of Search 75/13223/223 nickel,manganese, etc., and the bulk of the ferrophosphorus alloy comprisingessentially iron, wherein a bathvof said ferrophosphorus is established[56] References Cited containing at least one additive selected from theUNITED STATES PATENTS group consisting of carbon and silicon in amountseffective to reduce the solubility of phosphorus in said 1,265,0765/1918 Gray..-. ..75/132 bath thereby resulting i a substantial amountof 1,863,642 6/1932 Strmson .L phosphorus vapor which is removed andthen 1,836,618 12/1931 Pokorny ..23/223 recover 3,231,335 1/1966 Billset al. ..23/223 3,305,355 2/1967 Darrow et a1 ..23/108 X 2 Claims, 2Drawing Figures ,4 [#ea) 0/ JW/ZM on 4' P/mspfium: (o/1190f M FeP new 425002' 475004000 Alia/v05 2a P/essz/re 0, h s 3 M (a Q 1 a 0 /2 R Thisinvention relates to the dephosphorization of ferrophosphorus, wherebythe phosphorus may be recovered, and whereby a ferrous material ofimproved quality and value is provided having particular utility in theproduction of steel and cast iron.

Ferrophosphorus is produced as aby-product in the manufacture I ofphosphorus by the reduction of phosphate ores. Such ores exist in thewestern part of the United States, among other places, and comprisesubstantial amounts of P (e.g., 34 percent) in the form of dry calciumphosphate. The calcium content of the ore may range by weight from about30 to 50 percent expressed as CaO and contains in addition silica, ironoxide, and sometimes small amounts of oxides of vanadium, chromium,nickel and also manganese, as well as organic matter and clay. Theamount of vanadium as V 0 may range from about 0.05 to 1 percent byweight.

Generally speaking, the contained phosphorus is extracted by employingelectric furnace smelting techniques as described in US Pat. Nos.3,154,410 and 3,305,355. Broadly, this method comprises providing acharge of coke and ore and smelting the charge in an electric furnaceunder reducing conditions which cause the phosphorus to be reduced tothe elemental phase and at the temperature prevailing volatilized anddrawn off, condensed and recovered. The metallic oxides upon reductionform phosphides which pass through the slag and collect as a molten poolof fer-- rophosphorus at the bottom of the furnace. The slag containscalcium oxide, silica, alumina and the like. Periodically, the slag andferrophosphorus are tapped from the furnace.

The ferrophosphorus produced in the foregoing manner from Western oremay contain 20 to 30 percent by weight of phosphorus, up to aboutpercent vanadium, more generally 2 to 9 percent, up to about 8 percentchromium, and usually up to about 2 percent nickel, and the balanceessentially iron. The foregoing byproduct has found some use in thesteel industry as a ferrophosphorus (ferrophos) addition agent.

As is well known to those skilled in the art, ferrophosphorus is alsoproduced from other phosphate ores, such as those in the southern partof the United States, which do not contain all the elements recitedhereinabove, but which may contain relatively high manganese instead.Whatever the composition of the ferrophos, the market for it in the ironand steel industry has not been economically attractive. This has beenparticularly true of the ferrophos produced in the western part of theUnited States which is located far from the market, although it isrecognized as being intrinsically valuable.

Methods have been proposed for recovering the vanadium from Westernferrophos by passing oxygen through molten ferrophos at a temperaturewithin the range of about l,300 C to l,560 C whereby to concentrate thevanadium and chromium in the slag, which slag, containing as much as 15percent vanadium as V 0 is then processed to recover the vanadium.

One method proposed for recovering the vanadium (note US. Pat. No.3,154,410) is to grind the vanadium-rich concentrate (i.e., slag) in aball mill with 50 percent sodium chloride to a fineness exceeding 150mesh. The material thus prepared is charged to a multiple hearth furnaceand roasted at a temperature in the range of about 600 C to 800 C andthe roasted concentrate thereafter leached with water to recover thevanadium as sodium vanadate which is then recovered from solution byknown chemical methods.

However, the residual ferrophos remaining after slagging off thevanadium concentrate is still relatively high in phosphorus (e.g., 23percent) which has only the limited use as a source of phosphorus insteel making. Thus, the cost of the foregoing process is borne solely bythe recovered vanadium which puts a limit on the overall economics ofthe process.

It is known to treat high phosphorus iron in a destructive distillationprocess (US. Pat. No. 3,231,335) to lower the phosphorus content, butsuch processes have not been too efficient.

It would be desirable to provide a process in which the by-productferrous metal remaining after vanadium recovery also contains lowphosphorus so as to increase the uses and value of the by-productferrous metal and thus upgrade the economics of the vanadium recovery.It would furthermore be desirable to recover phosphorus in its elementalform, thereby greatly enhancing the economics of the operation.

We have now discovered a process whereby we can efficientlydephosphorize ferrophos to levels whereby the value and use of theferrous product remaining are greatly enhanced, while also recoveringelemental phosphorus. In addition, our process is unique in that it canbe applied to vanadium-containing ferrophos so that the recovery ofvanadium is rendered additionally attractive from an economic viewpoint.

It is thus the object of this invention to provide a process wherebyferrophos can be efficiently dephosphorized to levels whereby theresulting ferrous product is greatly enhanced in value and use.

It is a further object of this invention to provide a process whichpermits the recovery of phosphorus in the elemental form.

Another object is to provide a process for dephosphorizingvanadium-containing ferrophos in which vanadium is also recovered as aby-product.

These and other objects will more clearly appear from the followingdisclosure and the accompanying drawing wherein:

FIG. 1 is a flow sheet illustrating a preferred embodiment for carryingout the invention; and

FIG. 2 depicts a curve showing the effect of silicon additions on thedephosphorization of ferrophos.

Stating it broadly, the invention resides in a process for the vacuumrecovery of phosphorus from ferrophos containing by weight about 20 to30 percent phosphorus, up to about 15 percent vanadium (e.g. 2 to 9percent), up to about 7 percent chromium (e.g., 2 to 6 percent),possibly some nickel, small amounts of manganese, copper, etc., and thebalance essentially iron; In its more preferred aspects, the inventionincludes the separation of vanadium from ferrophos for subsequentrecovery thereof. In essence, the invention comprises establishing amolten bath of ferrophos with an amount of at least one addition agentselected from the group consisting of up to about 5 percent carbon andup to about 15 percent silicon by weight effective to reduce thesolubility of the phosphorus in said bath,

thereby resulting in a substantial amount of phosphorus vapor, leadingoff the resulting vapor, and then recovering elemental phosphorus bycondensation. Ad vantageously, the silicon addition may range from about2 to 10 percent by weight and, more preferably,

from about 5 to percent, and the carbon from about 2 to 5 percent.

The invention may be carried out as part of the oresmelting cycle in theelectric furnace, or,.alternatively, a bath of the ferrophos itself,after leaving the smelting furnace or, separately prepared, can-betreated in a holding furnace or a ladle enclosed in a hood to remove thephosphorus. In either case, provisions are made to establish a vacuumabove the melt. The treating temperature is minimally l,400 C, butpreferably in the range of 1,500 to l,600 C;

By vacuum, we mean that amount of sub-atmospheric pressure necessary toeffect evolution of gaseous phosphorus from the melt under theprevailing metallurgical conditions. As one skilled in the art willappreciate, during the evolution of phosphorus gas, the pressure in thesystem is determined by the phosphorus gas itselfpWhen 'the reactionhasbeen substantially completed and the rate of evolution of the phosphorushas declined, the pressure in the, system will ac-.

cordingly drop in proportion. The reaction is considered to be completedwhen the pressure in the system drops to below 10,000 microns, ofmercury e.g.

below 4,000 microns, and, more advantageously, belowv 2,000 microns.

ln carrying out the process in accordance with FIG. 1, a bath offerrophos is established and treated in accordance with steps A, B and Cas one embodiment of the invention, itbeing understood that variationsin the method may be employed in treating the ferrophosphorus within thebroad aspects of the invention. Step A A melt offerrophos is establishedin either an electric phosphorus smelting furnace provided witha'suitable vacuum system, or a ferrophos product previously produced isplaced eitherin the liquid or solid state into a vacuum furnace whereheat is applied during the treatment. A typical ferrophos charge .of2,000 pounds shown in FIG. 1 may comprise approximately 56 percent Fe,26 percent'P, 8 percent V, 6.5 percent Cr, 2 percent Ni and 1.5 percentof other elements. The ferrophos containing about 10 percent silicon ismaintained at a temperature of about l,550 C under a vacuum produced asdescribed hereinabove, in this case, 1,000 microns. The treatment iscarried out to produce depleted ferrophos containing about 5 percent P.As will be noted from Step A in FIG..1, 428 pounds P may be evaporatedfrom the melt to produce depleted ferrophos containing approximately 65percent Fe, 5 percent P, 9 percent V, 7 percent Cr, 2 percent Ni, 1 1percent Si and the balance other elements (X). Step B The depletedferrophos from Step A is then subjected to treatment in Step B by beingplaced into a converter vessel containing MgO as a slag-formingingredient in the amount shown together with iron oxide for the purposeof oxidizing the silicon in the depleted ferrophos.- In addition to theoxygen supplied by the iron oxide, more oxygen is provided by blowingair into the melt. This treatment causes vanadium, chromium and someiron tobe oxidized and to pass into a slag in-v dicated in StepB of FIG.1 which is then available for further treatment for recovering vanadiumor possibly chromium values. The residual metal, which is defined aslean depleted ferrophos, is still too high in phosphorus to have valueas melting stock in conventional steel melting practice. It is thensubjected to further treatment in Step C.

Step C Step C consists of the treatment of the lean depleted ferrophosin a basic bessemer converter in which the molten bath is blown with airunder a lime slag to remove the phosphorus to desirable low levels, suchas 0.3 percent. This may involve double slag practice. It is preferredthat the slag contain P 0 in sufficient concentration to make itattractive as raw material for recycle to the electric phosphorussmelting furnace. It is possible by the foregoing technique to produce afinal metal product containing about 2.4 percent Ni, 0.3 percent P, andthe balance substantially Fe with small amounts of residual elements.This material has commercial usefulness as a nickel-containing meltingstock for the production of alloy steels and irons.

As will be noted from the flow sheet, the treatment of 2,000 pounds offerrophos may result in a final metal product of about 1,700 pounds, aswell as 1,200 pounds of enriched vanadiferous slag, about 400 pounds ofelemental phosphorus, and 500to 600 pounds of recycle calcium phosphateslag.

The. effectiveness of silicon as an additive for reducing the solubilityof phosphorusin molten iron will be apparent from Table l which showsthe results obtained with silicon additions ranging from about 1 to. 10percent by weight of the ferrophos bath containing about 26 percentphosphorus after about 1 hour of treatment at vacuums below 1,000microns.

TABLE 1 Silicon by Wt. of Ferrophos Final P tions of silicon of about 5to 10 percent by weight based on ferrophos are particularly advantageousin lowering the phosphorus content.

TABLE 2 cartime carbon at Charge bon (petemp tempFinal Final Test FeP(grrce- (C) (h-. vacuum P No. (grams) ams) nt) ours) (microns) (percent)1 99 1 l 1500 2 650 14.36 2 98 2 2 1500 2 50 12.35 3 97 3 3 1500 2 l12.28 4 95 5 5 1500 2 275 13.00 5 1900 100 5 1600 l 180 14.3 6 1900 1005 1600 l 40 12.7 7 1900 100 5 1700 l 140 13.9 8 3800 200 5 1700 1 7513.3 9 I900 100 5 1500 l 70 15.6

As will be noted from the table, the best results are obtained withcarbon ranging'from about 2 to 5 percent by weight of the bath. Animportant consideration in the use of carbon in place of silicon is itsgreater economy. Carbon may therefore be preferred where it is desiredto recover phosphorus from ferrophosphorus and thereby improve theoverall yield of phosphorus from apatite in the electric furnaceprocess. Where there are present elements such as vanadium, chromium,nickel, manganese, etc. in sufficient concentration to warrant theirrecovery, it may be desired in order to facilitate metallurgicalprocessing to use silicon in preference to carbon, or a combination ofboth, to lower the phosphorus to the greatest possible extent asdescribed hereinbefore with respect to the FIG. 1 flow sheet.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and the appended claims.

What is claimed is:

1. In a method for recovering phosphorus from ferrophosphorus containingby weight about 20 to 30 percent phosphorus and containing optionally upto about 15 percent vanadium, up to about 7 percent chromium, smallamounts of other ingredients, and the balance essentially iron, theimprovement which comprises, establishing a molten bath of saidferrophosphorus at a temperature of at least about l,400 C under vacuumbelow 10,000 microns of mercury containing as an ad- I ditive about 5 to10 percent silicon effective to reduce further the solubility of saidphosphorus in said bath under said vacuum, thereby forming a substantialamount of phosphorus vapor, removing the resulting vapor whilemaintaining said vacuum and then recovering said phosphorus.

2. The method of claim 1, wherein the vacuum is less than 2,000 microns.

2. The method of claim 1, wherein the vacuum is less than 2,000 microns.